The present invention relates generally to testing systems for digital transmission lines and, more particularly, to a testing system for digital telecommunications transmission lines that allows for a metallic loopback of test data and a direct current ("DC") bypass. Many digital telecommunications transmission facilities include a central office which may transmit data, or "payload," signals over transmission lines to customer premises equipment. Typically, the payload is sent over the transmission lines differentially on two copper conductors, known as the Tip-Ring pair.
The Bell telephone system in the United States, for example, has widely utilized Digital "D" multiplexing pulse code modulation systems. A "D" channel bank, for example, commonly provides multiple DS-1 signals that are carried on a T-1 Transmission system. Each T-1 carries 24 Full Duplex channels on two pairs of exchange grade cables. One pair of cables is provided for each direction of transmission.
For clarification and simplification of terminology, the pair of cables carrying signals from the central office to the customer premises equipment is designated as a "transmit" line, and the pair of cables transmitting data from the customer premises equipment to the central office is designated as a "receive" line. This designation is made for convenience only; of course, when an observer (or testing technician) changes position from a central office to the customer premises, what used to be a "transmit" line becomes a "receive" line, and the "receive" line becomes a "transmit" line.
Payload signals are received by the telephone company and are transmitted, via the transmission lines, to a series of regenerative repeaters. Such repeaters are spaced along the transmission lines approximately every 6,000 feet. Each length of approximately 6,000 feet of cable may be designated as a "span."
The first repeater receives the data from the central office repeater, but, because of transmission line losses, jitter, noise, interference, and distortion, the signal will have degraded. The repeater recognizes the presence or absence of a pulse at a particular point in time and, if appropriate, regenerates a relatively clean, new pulse.
The regenerative repeater, or line repeater, is powered through the transmission cable itself to generate new pulses. Longitudinal, or common mode, direct current is applied supplied to the simplex lead to power the T-1 line repeaters. New pulses are transmitted by the repeater along more cable to either another line repeater or to the customer premises equipment.
As a part of its maintenance program, the telephone company may interconnect a test set (such as, for example, a Hewlett Packard 3787B test set) between the transmit and receive channels. The telephone company may then test the integrity of the lines and isolate (or "sectionalize") malfunctions along the transmission lines.
Such diagnostics may involve simply the ascertaining whether or not a particular span of cable provides continuity along its entire length. For example, an activating signal may be sent by the test set in, for example, the central office. The signal may designate a first repeater to "loopback" the signal from the transmit line to the receive line. Accordingly, a signal sent down the transmit line should then be received immediately thereafter at the receive line in the central office, if the lines to and from the repeater are continuous and the repeater has performed a loopback between the transmit and receive lines. In such a case, if continuity is proven, the test set may then instruct the repeater to connect the lines in standard transmission mode but, however, instruct the next most proximate repeater to loopback signals. Thereafter, if the test signal applied to the transmit line is not then also received at the receive line, the telephone company technician will know that the malfunction has occurred between the loopback of the first repeater and the loopback of the second repeater. The error in the line has then been sectionalized to a 6,000 foot interval rather than the entire length of the transmission line.
New generation DS1 fault location systems may perform even more complex diagnostics which, in addition to checking for discontinuities, can also look for transmission abnormalities such as bi-polar violations, "stress" pattern errors and, for example, provide information on the number of seconds of severely errored data that is transmitted along the lines. The types of performance evaluation messages possible are defined, for example, in the American National Standards Institute Specification T-1.403-1989. The telephone company wishes to be able to quickly and inexpensively isolate the location of, or sectionalize, the malfunction.
The craftsman also wishes to obtain true sectionalization of the transmission lines, and thus more precisely locate the difficulty. Importantly, the test equipment should not affect line performance, such that a line might perform differently with and without the test equipment applied to the line. For example, the stress patterns (occurrences of severely distorted information) may vary if the line is significantly altered in order to perform a sectionalization test.
Moreover, it is important that all portions of the transmission lines be subjected to a test when the telephone company performs a diagnostic test on a line's performance. There is typically a metal interface forming part of the physical connection between the repeater and the beginning (or end) of another span. The data looped back from the transmit to the receive line should be close to such metal, thus providing a "metallic" loopback. Such a metallic loopback thus, for example, may allow for the testing not only of the regenerative features of the repeater itself, but also of other circuitry, such as overload protection equipment that may be within the repeater. The overload protection circuitry can be damaged by lightening surges during lightening storms and extremely high voltages from shorted power lines.
Also, it is important that the components of the test system used in the field have relatively small physical size and weight. Moreover, a test system should be inexpensive to manufacture and install. Furthermore, the testing system should be reliable, so as to increase the reliability of the telephone system itself.
Unfortunately, many of the presently available performance monitoring systems are not ideally suited to the demands of the telephone companies. Many of the presently available systems do not allow a "metallic" loopback during testing. Such systems may allow for the testing of the regenerative aspects of the line repeater, but not of its overload protection circuitry.
Also, many available systems may, during testing, remove the DC power applied to spans and repeaters located farther away from the test set than the repeater which has been instructed to loopback signals. Such blocking of DC signals from the outlying spans and repeaters may affect the test results received during interrogation. Furthermore, prior art systems may require the installation of bulky, heavy, and expensive equipment in the field.